Data streaming: run starts: standardise on JSON schema for metadata structure

https://github.com/ess-dmsc/streaming-data-types/blob/master/schemas/pl72_run_start.fbs#L29 allows for metadata such as the user names and other data which we will need to put in the run start message.

This ticket is to document these so that the keys remain the same between whatever produces them( likely `kdae_control`) and any downstream consumers such as the journal appender started in #11

### acceptance criteria
- [ ] A JSON schema is added to this repo that sets the key names of basic fields we would like to the metadata. These should be the ones in the journal XML files.
---
Discussed Planning 08/01/2026 14:36